DC Current Reads Incorrect

Since you say you have 6 identical setups, you can troubleshoot:

1. If you move the shunt, does the limited current display follow it?

2. If you move the ABB signal converter(s), does the limited display follow it?

3. If you swap the 2 ABB signal converters on one of the setups, does the limited display follow them?

If so, you should be able to identify which one of them is the problem. If not, then the problem is something about the DCS input you are using (is that the Allen Bradley I/O & PLC?), and should be investigated with Allen Bradley's tech support.

Your testing so far has given you a lot of clues that the problem is at the DCS input, but the above should verify it for you.

>We changed the ABB converter but there was no change in the display

That says that the converter probably is not at fault, particularly because it's got 3 way isolation, they work OK on the bench, and two of them produce the same results connected to the DCS.

Apparently the local controller has control. Does that mean you can run a test on the DCS without waiting for an outage?

Why not disconnect the ABB's output at the DCS input, connect a simulator/calibrator to the DCS input and see what happens as the signal gets above 4.0Vdc?

If the DCS AI is working correctly and the point is scaled correctly, then 5.0Vdc from the simulator should give a 30KA reading.

If a 5.0Vdc simulator output clamps the DCS reading at 24.5KA, then either someone changed the point scaling on the DCS input (assuming it ever worked properly) or the AI is faulty.

I smell ground loop. Try a ground loop isolator to the DCS.

The CC-U/STD is a loop isolator: 3 way: input/output/power.

We had a small window to shut down and test the converters. We disconnected the shunt inputs and applied a calibrator 0 - 50mv to the input of each. The convertor for the local display went from 0 to 5volts, the DCS convertor stopped at 3.6 volts. We switched the outputs so that the local was feeding the DCS channel and it also stopped at 3.6. We thought at that point there was a problem with the input channel on the card and replaced the card.

Our time was up at that point. when we started up it was looking good until we got to around 28000A then the channel stopped following the current. So it is doing the same thing now but at a bit higher level than before. Anything else we could check into?